Zeolite Encaged Cu(Histidine) Complexes as Mimics of Natural Cu Enzymes

Abstract

The most abundant, active, and selective catalysts i n nature are enzymes. Their operational domain is. however, relatively narrow as far as temperature and solvent are concerned. Enzyme mimicking is the building of the active center of enzymes (or an analogy of it) into a matrix which allows a larger operational temperature domain and a broader spectrum of solvents. In the case of inorganic matrices. zeolites are the obvious choice because the pore diameters and pore geometry introduce shape selectivity in the reactions. Up to now, the active center of the enzyme has been mimicked by construction of robust, rigid. and stable transition metal ion complexes in zeolite cavities and channels.". 21 The most prominent examples are phthalocyanine and bipyridine complexes.[" 41 However. these complexes can only be synthesized in situ and this procedure is time-consuming. Moreover, the turnover numbers in catalytic oxidations are relatively low.Here, we report for the first time on the immobilization of Cu(histidine) complexes in zeolites by a simple ion exchange procedure. Amino acids. like histidine, are the key building units of natural enzymes; therefore, the obtained inorganic enzyme has an active center, very close to that of its natural enzyme counterparts. In addition, the complexes are built in by a simple ion exchange procedure. Cu2+ was chosen as the transition metal ion. because its complexes are stable ['] and easily characterized by electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS) in the UV/VTS/NIR region.161 Furthermore, copper proteins play a key role in both plant and animal physiology. for example hemocyanin is the oxygen-carrying protein in the hemolymph of molluscs and arthropods; ascorbate oxidase catalyzes the oxidation of L-ascorbate and galactose oxidase. which catalyzes the oxidation of galactose and many other substrates, like aliphatic and aromatic alcohol~.['-"~ Cu(histidine) complexes are typically prepared in bidistilled water with a histidine:Cu2 + ratio of 5 : 1 . By using an increasing amount of preformed [Cu(His),]+ complex at pH 7.3 in bidistilled water['21 together with a zeolite Na-Y, we have measured the amount of released Na'. together with the amount of Cu" taken up by the zeolite material. The results are presented in Figure 1. Although there is a considerable spread of points, the data show unambiguously that ion exchange is operative. The slope of the straight line (7Na'iCu '') indicates that besides [Cu(His)J+, His' is exchanged too. Chemical analysis indeed confirms that the histidine:Cu2+ ratio on the solid is 6 : l . Thus at the low exchange levels investigated.'' 31 for each [Cu(His),]' exchanged, four His+ cations are co-exchanged. The fact that on the average seven N a + ions are released and not six is indicative for charge compensation by H + ; thus some residual acidity is expected on the solids.

Zeolite Framework Stabilized Copper Complex Inspired by the 2-His-1-carboxylate Facial Triad Motif Yielding Oxidation Catalysts

“…[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] In most cases, the aim is to improve the separation of the catalyst from the products or the selectivity of a catalytic reaction. A zeolite supercage can also be seen as a protective structure around the active center of the catalyst, similar to the protein mantle surrounding the active site of enzymes.…”

Zeolite-Encapsulated Copper(II) Amino Acid Complexes: Synthesis, Spectroscopy, and Catalysis

“…10 Three subjects will be studied systematically: (1) the ion exchange of preformed Cu(AA) n m+ complexes in zeolites, (2) the spectroscopic fingerprinting of immobilized Cu(AA) n complexes, and (3) the catalytic behavior of immobilized Cu(AA) n m+ complexes. These data allow a detailed discussion of the coordination chemistry of zeolitic Cu(AA) n m+ complexes.…”

Clay intercalated Cu(II) amino acid complexes: synthesis, spectroscopy and catalysis

“…The catalytic performances of the [Cu(histidine)2+]-saponite catalyst are summarized in Table 3 and compared with those of zeolite occluded Cu(histidine) complexes. Although the saponite-based catalyst is active in the oxidation of 1-pentanol, benzylalcohol and cyclohexene, the conversion is always lower than in the case of the zeolite occluded complexes (Weckhuysen et al, 1995). Furthermore, the catalyst cannot be easily removed by centrifugation from the reaction mixture after reaction.…”

“…The reactivity of the immobilized complexes has been studied in liquid phase oxidation reactions in the presence of peroxides. The results are compared with those previously obtained using zeolites (Weckhuysen et al, 1995(Weckhuysen et al, , 1996 …”

“…The reactivity of the immobilized complexes has been studied in liquid phase oxidation reactions in the presence of peroxides. The results are compared with those previously obtained using zeolites (Weckhuysen et al, 1995(Weckhuysen et al, , 1996 EXPERIMENTAL Synthesis Saponite with a CEC of 0.67 mEq g-l and a particle size of ~<2 I.tm was used for cation exchange with preformed Cu(AA)n complexes. Solutions of Cu(AA)n were prepared by mixing Cu(NO3)2.3H20 (Merck; AR) and AA (L-histidine and L-(+)-lysine, both from Janssen Chimica) at a AA:Cu 2+ molar ratio of 5 in distilled water ([Cu 2 § = 1 x 10 -3 M).…”